4. National Air Quality Strategy Pollutants

The National Air Quality Strategy was published in 1997 and sets out a framework of standards and objectives for the pollutants of most concern (SO₂, PM₁₀, NO_x, CO, lead, benzene, 1,3-butadiene and tropospheric ozone) with the aim of reducing the number and extent of episodes of air pollution, both in summer and winter. It identifies air quality standards for 8 priority pollutants based on the recommendations of EPAQS or WHO guidance where no EPAQS recommendation exists. Specific objectives set out the degree of compliance with each standard to be achieved by the year 2005.

Local authorities in the UK have a duty, under the 1995 Environment Act: Part IV, to review and assess air quality in their areas. The Air Quality Regulations 1997 define a staged process of review and assessment on the basis of guidance provided by DETR. The first stage primarily involves the collection of existing data on air quality measurements and emission sources for the 8 pollutants of interest in the NAQS. These data are then used to define whether there is likely to be an air quality problem in 2005, the attainment date for the Strategy. The second and third stages require the use of increasingly sophisticated monitoring and modelling tools to identify hotspots of pollution and to determine whether these will meet the relevant air quality objectives.

The NAEI is being used as an important source of data for the compilation of appropriate local inventories. Table 4.1 summarises the total 1996 emissions of the 8 priority pollutants covered by the NAQS.

Table 4.1 Total UK Emissions of NAQS Pollutants

Table 4.2 Location of Emissions and Discussion of NAQS Pollutants

Historically, interest in particulate matter focused mainly on smoke which can cause health problems especially in combination with other pollutants. The classic example was emissions of smoke and sulphur dioxide leading to the London smogs in the 1950s and early 1960s when several thousand excess deaths were recorded. Smoke emissions have fallen significantly as a result of the Clean Air Act eliminating domestic coal combustion in many urban areas. However, there is increasing interest in the measurement of fine particulates, such as those arising from the combustion of diesel in the transport sector, and aerosol concentrations in the atmosphere from other sources which may have harmful effects. Recent epidemiological evidence is also linking concentrations of particles in the atmosphere with human health effects. Indeed, current ambient mass concentrations are thought to be sufficient to lead to increased mortality and morbidity (EPAQS, 1995).

For many years the monitoring of particulate levels was based on the measurement of Black Smoke. Levels were estimated using a simple non-gravimetric reflectance method in which air is sampled through a filter and the resulting blackening measured. The method was calibrated for domestic coal smoke. When most of the emissions come from coal combustion the blackening should be roughly proportional to the mass concentrations. In the 50s and 60s, domestic coal combustion was the dominant source of black smoke and hence this method gave an indication of the concentration. The NAEI estimates of black smoke emissions were extended in 1988 to include emissions from all fuel combustion. Prior to 1988 only emissions from coal combustion had been estimated and published in the DOE Digest of Environmental Statistics. However, there have been no recent emission measurements to confirm these factors are still accurate.

Smoke from different sources has a different blackening effect and so there is no simple relationship between black smoke and the mass of particulate emissions. For example, typically diesel emissions have a blackening effect three times greater, mass for mass, than coal emissions, while petrol emissions are effectively an order of magnitude less. As black smoke is such a poor indicator of the concentrations of particulates in the atmosphere current interest and the NAQS is focused on PM₁₀ (particulate matter less than 10mm) and smaller size fractions (EPAQS, 1995).

Particles can vary widely in size and composition. Particles larger than about 30 mm fall rapidly under gravity and those larger than about 100 mm fall out of the atmosphere so rapidly they are not usually considered. At the other end of the size scale are particles less than a thousandth of a mm which are so small they do not fall under gravity appreciably, but coagulate to form larger particles that then are removed from the atmosphere.

The US PM₁₀ standard was a monitoring standard designed to measure the mass of particles less than 10 mm in size (more strictly, particles that pass through a size selective inlet with a 50% efficiency cut-off at 10 mm aerodynamic diameter). This corresponds to the International Standards Organisation thoracic convention for classifying those particles likely to be inhaled into the thoracic region of the respiratory tract. The epidemiological evidence on the effects of particulates shows good correlation between PM₁₀ concentrations and mortality or morbidity (EPAQS, 1995). Therefore there is increasing interest in PM₁₀ in the UK and around the world. PM₁₀ measurements are now being made in the UK (Bower et al, 1994, 1995 a&b) and their emissions have been included in the NAEI since 1995.

For completeness the following sections present emission estimates for Black Smoke, PM₁₀ and some preliminary estimates for PM_2.5. PM_1.0 and PM_0.1 are also discussed.

4.2.2 Black Smoke

The UK emissions of black smoke are shown in Table 4.3. Emissions have declined over the period 1970-1996 by 68%. The main reason for the reduction is the decline in the use of solid fuels in the domestic sector. Up until the late eighties, domestic emissions were the most important contribution to the total, however since then emissions from road transport have dominated and currently account for 58% of the total. Road transport emissions have risen by a factor of 2 since 1970 and these are largely composed of emissions from diesel engines. Road transport emissions have declined since 1992 due to stricter regulations on diesel engines. Emissions from off-road sources have been estimated from estimates of the amount of diesel oil and petrol consumed by these machines. The emissions account for around 9% of the UK total. The small decline in agricultural emissions in 1993 is due to the banning of stubble burning.

Table 4.3 UK Emissions of Black Smoke by UNECE¹ Source Category and Fuel (kt)

It is likely that emissions of black smoke have fallen more than is indicated by these estimates as a result of improved abatement measures on large combustion plant and stricter regulation on diesel engines. However, little data is available on these control measures, including their effect on the blackening effect of particulates which is crucial to the black smoke inventory, and only the diesel emission reductions have been estimated.

4.2.3 PM₁₀

PM₁₀ in the atmosphere arises from two sources. The first is the direct emission of particulate matter into the atmosphere from a wide range of sources such as fuel combustion, surface erosion and wind blown dusts and mechanical break-up in, for example, quarrying and construction sites. These are called 'primary' particulates. The second source is the formation of particulate matter in the atmosphere through the reactions of other pollutants such as sulphur dioxide, nitrogen oxides and ammonia to form solid sulphates and nitrates, as well as organic aerosols formed from the oxidation of VOCs. These are called 'secondary' particulates. This inventory only looks at primary sources. For further information on secondary particulate see the third Quality of Urban Air Review Group report (QUARG, 1996).

There is currently a programme sponsored by DETR and SMMT to measure PM₁₀ emissions from road transport. This programme has developed measurement techniques and aims to produce improved PM₁₀ emission factors, particulate characterisation and size distribution.

The main sources of primary PM₁₀ are briefly described below:

• Road Transport. All road transport emits PM₁₀. However diesel vehicles emit a greater mass of particulate per vehicle than petrol-engined vehicles. Emissions also arise from brake and tyre wear and from the re-entrainment of dust on the road surface. This final contribution, from re-entrainment of dust, is not yet included in the inventory. This is because of the lack of a suitable estimation methodology for re-entrainment on roads in the UK.
  
• Stationary Combustion. Domestic coal combustion has traditionally been the major source of particulate emissions in the UK. Now, its use has been restricted in the UK due to the Clean Air Acts so other sources are more important nationally. However, domestic coal can still be a significant source in some smaller towns and villages, in Northern Ireland and in areas associated with the coal industry. Other fuels emit PM₁₀, but emissions from gas use are very small. In general, particles emitted for combustion have smaller sizes than those from other sources.
  
• Industrial Processes. These include bulk handling, construction and quarrying. These sources are all difficult to quantify due to the nature of the sources and to the lack of appropriate UK measurements. Usually a substantial fraction of the particles from these sources is larger than 10 mm but the large quantities emitted ensure that the fraction less than 10 mm is still substantial.
  

Emissions of PM₁₀ are shown in Table 4.4 and Figure 4.2. Emissions of PM₁₀ from the UK have declined since 1970. This is due mainly to the reduction in coal use. Domestic emissions have fallen from 216 tonnes (40% of the total emission) in 1970 to 30 tonnes (14%) in 1996.

The geographical disaggregation of emissions is shown in Figure 4.3. There is a clear distinction between the important sources in rural and urban areas. Indeed, many of the sources do not occur inside towns and cities. While road transport accounts for only 25% of national emissions, it can account for up to 80% of primary emissions in urban areas such as London (Buckingham et al., 1997)

Figure 4.2 UK Emissions of PM₁₀

Figure 4.3 Map(s) of PM10 Emissions

Table 4.4 UK Emissions of PM₁₀ by UNECE¹ Source Category and Fuel (kt)

Inventories for PM_2.5, PM₁ and PM_0.1 have been estimated from the PM₁₀ inventory and the mass fractions in these size ranges available for different emission sources and fuel types. A total of 33 different size distributions covering PM_2.5 and PM₁ emissions from different source sectors were taken from the USEPA (1995) as being applicable to sources in the UK; a fewer number of sectors with size fractions in the PM_0.1 range were available from the study by the TNO Institute of Environmental Sciences, Energy Research and Process Innovation in the Netherlands for the Dutch National Institute of Public Health and Environment (RIVM) (TNO, 1997) who produced a particulates emissions inventory for Europe. In general, combustion processes emit a higher proportion of fine particles (<2.5µm) than mechanical sources such as quarrying and construction. Gaseous fuels also tend to emit finer particles than petroleum and solid fuels.

Each of the detailed source sectors for which a PM₁₀ emission is estimated (a total of 139 individual sectors and sub-sectors) were allocated an appropriate size distribution and used to calculate emission inventories for PM_2.5, PM₁ and PM_0.1. The results are shown in Tables 4.5-4.7 in the same format as for the PM₁₀ inventory.

Figures 4.4-4.6 show trends in emissions of each particle size by source sector. The results show a general decline in emissions of each particle size since 1970, but with a slow down in the rate of decline as the particle size decreases. Between 1970 and 1996, UK emissions of PM₁₀ fell by 60%, whereas emissions of PM_2.5 fell by 53%, PM₁ by 51% and PM_0.1 by 34%. Also, there is a gradual change in the relative source contribution with particle size. This is illustrated more clearly in Figure 4.7 which shows the percentage contribution of each sector to PM₁₀, PM_2.5, PM₁ and PM_0.1 emissions in 1970 and 1996. Road transport becomes an increasingly important sector as the particle size decreases. In 1996, it accounted for 25% of PM₁₀ emissions, but 60% of PM_0.1 emissions. Emissions from non-combustion sources show the opposite trend.

Table 4.5 UK emissions of PM_2.5 by sector (ktonnes) estimated for the mass fraction of particles below 2.5 µm in each sector in the PM₁₀ inventory

Table 4.6 UK Emissions of PM₁ by Sector (ktonnes) Estimated for the Mass Fraction of Particles below 1 µm in each Sector in the PM₁₀ Inventory

Table 4.7 UK Emissions of PM_0.1 by Sector (ktonnes) Estimated for the Mass Fraction of Particles below 0.1 µm in each Sector in the PM₁₀ Inventory

Figure 4.4 UK emissions of PM_2.5

Figure 4.5 UK emissions of PM₁

Figure 4.6 UK emissions of PM_0.1

Black smoke emissions are less accurate than those for SO₂ due to the nature of the measurement and in particular, the uncertainties in the blackening effect of particle emissions resulting from the combustion of different fuels. Accuracy is likely to be in the range +/-20-25%.

Although the primary emissions inventory for PM₁₀ is continuously being improved, the uncertainties in the emission estimates must still be considered high. These uncertainties stem from uncertainties in the emission factors themselves, the activity data with which they are combined to quantify the emissions and the size distribution of particle emissions from the different sources. There is also the possibility that not all the sources that exist have been considered and some sources which are known to exist have not been quantified because of the lack of relevant data for estimating the emissions.

Emission factors are generally based on a few measurements on an emitting source which is assumed to be representative of the behaviour of all similar sources. Emission estimates for PM₁₀ are based whenever possible on measurements of PM₁₀ emissions from the source, but sometimes measurements have only been made on the mass of total particulate matter and it has been necessary to convert this to PM₁₀ based on the size distribution of the sample collected.

It is not possible to quantify the accuracy of the national emission estimates, but it is possible to give a qualitative indication of the overall reliability of the estimates and to rank them by source sector. In order of decreasing reliability of emission estimates, the ranking by source is broadly:

1) Road transport
2) Stationary combustion
3) Industrial processes
4) Mining & quarrying and construction.

The most reliable emission estimates are from diesel cars and are based on a number of detailed measurements. Contributions from sources such as mining and quarrying and construction are subject to great uncertainty.

Although the major sources are thought to be included in the national PM₁₀ inventory, there are still some sources which have not been included either because of lack of data on emission factor measurements for conditions pertinent to the UK or because of lack of appropriate activity data. Sources that are notably missing from the inventory are:

• Entrainment of road dust. Whilst USEPA data exists for calculating emissions from this source, they are not considered appropriate for UK road conditions. Further measurements of dust resuspension on UK-typical roads are required to provide data for emission inventory application. Until then, resuspension of road dust remains a major area of uncertainty in the UK particulate emission inventory.
  
  
• Agricultural sources. Emissions from ploughing and harvesting activities may, in particular, be significant sources. Certain livestock farming activities may also be important. Emission factors relevant to UK agricultural activities are not available to estimate emissions from these sources at present. Further measurements of particulate emissions from different agricultural activities in the UK are required to provide data for emission inventory applications.
  
  
• Biological sources. Emission factors are not currently available for this source.